Light emitting diodes (LEDs) have advantages of small size, long life, and low power consumption. Therefore, LEDs are expected to be the leading next generation light source, and the light emission intensity of LEDs is predicted to increase further. However, as the light emission intensity of LEDs increases, heat generated by the LEDs increases. When heat generated by LEDs increases, the advantages of the LEDs may not be fully exploited. Therefore, the heat generated by the LEDs must be dissipated efficiently. In order to utilize light emitted from LEDs efficiently, it is important to increase the reflectance of a substrate on which the LEDs are mounted. It is also important that the substrate is not significantly degraded or discolored by the light and heat emitted from the LEDs. Furthermore, in the case where the LEDs are used outdoors, it is important that the substrate is not significantly degraded or discolored even if it is exposed to sunlight for a long period of time.
Conventionally, various substrates have been proposed as substrates on which elements such as LEDs are to be mounted (see, for example, Patent Literatures 1 to 4).